Polyacrylate-based PSAs have already been known for more than 40 years. Within this period they have proven themselves in a multiplicity of different applications, and have become established accordingly. As compared with the PSAs that are likewise widely utilized in self-adhesive tapes that are based on rubbers (e.g. natural rubber or styrene-butadiene rubber) or based on styrene block copolymers (SIS, SBS) they possess numerous advantages. These include their excellent UV and light stability, high resistance to thermooxidative ageing, custom-tailorable polarity and, typically, water-clear transparency. In polyacrylate PSAs, moreover, there is generally an inherent possibility for crosslinking of the polymer chains, a possibility which is generally exploited; as a result these PSAs even at relatively high service temperatures possess good cohesion and hence a high level of temperature stability of the bonds. A further advantage is that polyacrylate PSAs already possess pressure-sensitive tack per se, in other words without additional additives, such as tackifying resins or plasticizers, for example.
New applications, especially industrial applications, are imposing ever more extensive requirements on the performance of polyacrylate PSAs. With the systems available to date, meeting such requirements is in many cases very difficult and in others completely impossible.
Conventional approaches to controlling the properties of polyacrylate PSAs include the choice of identity and quantity of the comonomers employed, the adjustment of molar mass and molar mass distribution in the polymers, and the mode and extent of crosslinking of the polymers. The aforementioned parameters allow the profile of adhesives properties to be controlled with selectivity and precision.
It is found in this context that the comonomers available industrially which can be employed economically for polyacrylate PSAs are limited. Increasing restrictions are coming about, moreover, as a result of progressively tightened statutory regulations. Thus, for example, vinyl acetate and acrylamide have become two relatively objectionable base materials.
In order to meet the increasing requirements made of polyacrylate PSAs the more recent past has seen targeted development of polymerization processes for controlling the molecular weight distribution (DE 100 30 217; DE 100 36 801; DE 101 49 084). Polyacrylates synthesized with such polymerization processes can be utilized with advantage for applications which include pressure-sensitive adhesives that can be coated from the melt. The achievable potential for improvement, however, is limited.
Another path taken to get to improved products involves the possibility of selective synthesis of block copolymers (I. W. Hamley, The Physics of Block Copolymers, 1998, Oxford University Press, Oxford). As a result of chemical coupling of thermodynamically incompatible polymer blocks, such block copolymers exhibit microphase separation: that is, thermodynamically compatible polymer blocks associate while thermodynamically incompatible polymer blocks segregate into spatially separate regions, but without macroscopic phase separation. The result, depending on composition, are phases of different structure. Block copolymers utilized at present in PSAs typically possess two or more polymer blocks of high softening temperature (also referred to below as hard blocks; realized by means of a correspondingly high glass transition temperature or a correspondingly high crystallite melting temperature) and at least one block of low softening temperature (also referred to below as soft block). The composition in systems employed to date has been chosen so that the phase formed by the soft blocks forms a continuous matrix within the PSA, thereby endowing the system with the possibility of PSA properties. The polymer blocks which soften at high temperature associate or segregate to form phase regions (domains) which are typically approximately globular, which are present in dispersion in the continuous matrix of the soft phase and which below their glass transition temperature or crystallite melting temperature act as physical crosslinking points (G. Holden, N. R. Legge, R. P. Quirk, H. E. Schroeder (eds.), Thermoplastic Elastomers, 2nd Ed., 1996, C. Hanser Verlag, Munich). Advantages of PSAs based on such block copolymers include, for example, the possibility of realizing very high shear strengths.
A disadvantage of the aforementioned block copolymers is that in the case of their solvent-free processing the processing temperatures are typically situated well above the softening temperature of the hard block domains (in the case of hard blocks which solidify glassily the required coating temperatures are customarily above—in some cases at least about 30 K to 50 K or even further above—the glass transition temperature (Tg) of the hard block domains) in order for the melt viscosity and/or elasticity to be sufficiently low.
A further disadvantage is the fact that the thermal load-bearing capacity of PSAs based on abovementioned block copolymers, crosslinked physically by way of the hard block domains, is markedly limited as a result of the softening of the hard block domains at high temperatures.
A disadvantage of the known block copolymers comprising hard and soft blocks is the fact, moreover, that the only phase structures obtainable with them that can be used for PSAs are those wherein the hard block phase is dispersed in the form of approximately globular associations in the continuous soft phase of the polymer block of low softening temperature. Phase structures comprising prolate, i.e. uniaxially elongated (e.g. rodlet-shape), oblate, i.e. biaxially elongated (e.g. layer-shaped) or three-dimensionally disposed associations of the hard phase, which are typically formed at relatively high hard block concentrations, are unsuitable for the realization of PSAs, since such systems lack the sufficiently high pliability and/or lack a sufficiently low deformation modulus and so do not meet, for example, the Dahlquist criterion important for pressure-sensitive tack. The wide diversity of phase structures available for block copolymers (see e.g.: H. G. Elias in “Makromoleküle”; Wiley-VCH, 6th Edition 2001, Volume 2, section 8.5.2; I. W. Hamley, The Physics of Block Copolymers, 1998, Oxford University Press, Oxford) hence remains closed for PSAs.
A further disadvantage of known block copolymers is that in order to obtain physical crosslinking and hence in order to realize sufficient cohesion there must be at least two spatially separated polymer blocks of high softening temperature. Diblock copolymers consisting of only one hard block and one soft block are therefore of only limited suitability as a sole polymer component for use in PSAs, especially if high shear strengths are called for.
Block copolymers known correspondingly are thus severely restricted in their structure, and control possibilities for PSAs are limited accordingly.